The present invention relates to the art of turbomachines and, more particularly, to a turbomachine and a method for enhancing power efficiency in a turbomachine.
Modern turbomachines achieve higher efficiencies and higher power output through the use of high adiabatic compression ratios of 20:1 or greater. High compression ratios lead to high efficiency and increased power output. Adiabatic compression is achieved using high efficiency axial flow compressors and is invariably accompanied by a temperature rise that leads to reduced compression efficiency. In order to reduce this temperature rise, air pre-cooling devices siphon off heat and thus partially offset any efficiency gains. Also, partially compressed air is extracted for cooling hot gas path components of combustion turbines. The work going into producing the cooling air is not transferred to the turbine and thus represents a further loss of efficiency. To address this lost work and increase power output, many modern day turbomachines employ external compressors that pump in more air or supply steam injection at a compressor discharge (CDC) diffuser exit or downstream of the diffuser exit. However, external compressors represent additional power consumers and thus create efficiency losses. While steam injection enhances heat transfer rates and boosts power output, high quality high pressure (HP) steam is required. The use of high quality HP steam has a negative impact on efficiency resulting from useful bottoming cycle heat loss. Moreover, by injecting the steam at the CDC diffuser exit or downstream thereof, airflow cooling offsets any gains that may be achieved in compressor efficiency. Also, injecting steam at this particular point in the turbomachine requires efficient combustion chamber mixing before ignition in order to mitigate any combustion degradation.